This invention relates to a method of manufacturing a bag with a reinforced opening.
A multi-layered bag with side folds whose opening is to be closed by sewing has been manufactured by a method comprising the steps of superposing two or more elongated webs of the bag blank, folding back the respective side edges of the superposed webs, bonding these respectively folded back side edges to form a continuous tubular body, successively cutting the tubular body into sections (each having the length of one bag), and closing one end of each section by sewing.
These bags usually comprise two or more superposed layers and have satisfactory strength and capability of preserving their shapes for many applications.
However, it is a recent trend to decrease the number of superposed layers for the purpose of saving the material. As a result of my investigation, even when the number of the superposed layers is decreased, it was found that the strength of the body portion of the bag is sufficient for most applications but there still remain problems regarding the opening of the bag.
One problem involves insufficient stiffness of the opening due to shortage of the rigidity of the opening. For this reason, when the opening is closed by an automatic packing machine after filling the contents in the bag, the workability of the machine decreases. More particularly, if respective layers were neatly aligned at the opening after the contents have been packed in the bag, sewing of the opening could be performed without any trouble. Actually, however, since the respective layers are not neatly aligned at the opening, the percentage of rejects increases.
Another problem involves an extreme decrease in the strength of the opening which is fatal to certain applications. To reinforce the opening of multi-layer bag with side folds, a reinforcing sheet of paper 30 was applied to the opening of the bag as shown in FIG. 5 and then secured thereto by sewing. The reinforcing sheet of paper does not extend to the side fold 32 and when the folds are expanded outwardly by the packed contents, the strength of the bag decreases at the opening.